Lithium ion batteries are one of the most popular types of rechargeable batteries with one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. Lithium-ion batteries are growing in popularity for many applications due to their high energy density. Such applications comprise cellular phones, notebook computers, and vehicles like electro-bicycles and cars.
The three primary functional components of a lithium ion battery are an anode (the negative electrode during discharge) that contains a material that is oxidized during discharge of the battery; a cathode that contains a material that is reduced during discharge of the battery; and an electrolyte that provides for transfer of ions between the cathode and anode. For all three functional components a variety of materials may be used.
One type of lithium batteries uses “insertion” cathodes and anodes. Such batteries are referred to as “lithium ion” batteries. Both the anode and cathode are materials into which and from which lithium can migrate. The process of lithium moving into the anode or cathode is generally referred to as intercalation, and the reverse process, in which lithium moves out of the anode or cathode may be termed deintercalation. When a cell is discharging the lithium is extracted from the anode and inserted into the cathode. When the cell is charging, the reverse process occurs, i.e. lithium is extracted from the cathode and inserted into the anode. Therefore, insertion or “intercalation” electrodes contain materials having a lattice structure into which an ion can be inserted and subsequently extracted. Rather than chemically altering the intercalation material, the ions slightly expand the internal lattice lengths of the compound without extensive bond breakage or atomic reorganization.
Insertion anodes of conventional commercial lithium-ion cells are made from carbon, or rather graphite although other anode materials are also known, e.g. lithium chalcogenides like lithium oxide, and/or investigated.
A variety of materials have been suggested for use as cathode active materials in intercalation cathodes of lithium ion batteries suitable for multiple intercalation and deintercalation cycles. Such materials include, for example, MoS2, MnO2, TiS2, NbSe3, LiCoO2, LiNiO2, LiMn2O4, V6O13, V2O6, CuCl2, transition metal oxides, such as those of the general formula LixM2Oy, are among those materials preferred in such batteries having intercalation electrodes. Other materials include lithium transition metal phosphates, such as LiFePO4, and Li3V2(PO4)3.
During charging, lithium from the cathode is transferred to the anode where it intercalates while during discharging the lithium is transferred from the anode to the cathode where it intercalates. This back-and-forth transport of lithium ions (Li+) between the anode and cathode during charge and discharge cycles has led to these cells as being called “rocking chair” batteries.
The third functional component, the electrolyte, is a lithium salt in an organic, typically aprotic and water-free solvent.
Depending on the choice of material for the anode, cathode, and electrolyte the voltage, capacity, lifetime, and safety of a lithium ion battery can change dramatically.
Useful work can only be extracted if not only lithium ions are moved but also electrons flow through an external circuit. Therefore the ease of electron removal and receipt are relevant.
Recently several new and/or improved electrode materials have been developed, some of them being based on electronically active nanoparticles in combination with a conductively filled binder and/or with an electrically conducting binder, for weight reduction also in form of nanoparticles. Patent applications on such materials are e.g. EP 2 228 854 A1 and EP 2 287 946 A1.
It is also already known to combine more than one electrochemically active material (EAM) for producing an electrode, e.g. a cathode. Such combination materials are described in e.g. U.S. Pat. No. 3,981,748, U.S. Pat. No. 7,811,707, and U.S. Pat. No. 7,811,708.
U.S. Pat. No. 3,981,748 discloses combinations of silver phosphate (Ag3PO3) and silver chromate (Ag2CrO4) and lithium phosphate (Li3PO4) and silver chromate as cathode materials in lithium batteries. The combination with silver chromate is made for reducing the expansion found for pure silver phosphate resulting in a different voltage.
U.S. Pat. No. 7,811,707 and U.S. Pat. No. 7,811,708 of the same applicant are closely related and deal with the problem of enhancing the battery safety. As solution they propose compositions comprising at least one of a lithium cobaltate and a lithium nickelate; and at least one of a manganate spinel and an olivine compound.
U.S. Pat. No. 7,041,239 B2 discloses binary or ternary blends intended for improving electrode characteristics such as cycling capacity, capacity retention, operating temperature characteristics and voltage profiles. The materials are an olivine type material, an alkali metal transition metal oxide with the transition metal being different from Mn and a n alkali metal manganium oxide.
In general, a desirable cathode material must exhibit a high free energy of reaction with lithium, be able to intercalate a large quantity of lithium, maintain its lattice structure upon intercalation and deintercalation of lithium, allow rapid diffusion of lithium, afford good electrical conductivity, not be significantly soluble in the electrolyte system of the battery. Preferably such material can readily and economically be produced.
Many of the cathode materials known in the art lack one or more of these characteristics. As a result, for example, many such materials are not economical to produce, afford insufficient voltage, have insufficient charge capacity, or lose their ability to be recharged over multiple cycles.
Therefore, there is still a need for better materials with e.g. higher capacity and/or more recharging cycles.